This invention relates to methods for depositing coatings on suitable substrates utilizing a radio frequency (rf) powered radial-flow reactor in which laminar flow of reactant gases in a radial direction over a surface is utilized.
The reliability of semiconductor devices, particularly metal oxide silicon semiconductor devices (MOS), is largely a function of the manner in which these devices are passivated and how the completed devices are isolated from the environment. Two of the main problems associated with semiconductor devices are moisture and sodium contamination. Both these elements tend to attack unprotected semiconductor devices and can lead to a failure of the device. Some passivation layers are utilized to protect the surface of the semiconductor device from handling during the fabrication thereof and to provide an electrical isolation barrier. Generally, devices with this kind of passivation layer are packaged in hermetically sealed units in order to prevent sodium and moisture damage.
The relatively high cost of hermetic packaging and the testing associated therewith present a serious economic problem. Many of today's integrated circuit chips cost significantly less than the hermetic packages used to house these chips. It has become very desirable to provide a covering film which will not only protect semiconductor chips from mishandling, but will insulate the chips against moisture and undesirable impurities such as sodium. Semiconductor chips utilizing such a protective film could be placed in relatively inexpensive packages (i.e., nonhermetic plastic packages). This would significantly reduce the total cost of the packaged chip.
One commonly used passivation layer on aluminum metallized semiconductor chips is a phosphosilicate glass film which acts as a getter for sodium. One of the problems with this film is that it has a tendency to react with moisture and from phosphoric acid as a result thereof. This tends to corrode the aluminum metallization. One other problem is that the metallized films deposited subsequent to the film's deposition do not tend to adhere well to the protective film and, therefore, both moisture and sodium damage can occur in the areas of poor adherence. These films are typically deposited by a chemical vapor deposition (CVD) process. Physical steps occurring on the semiconductor chip surface can be poorly covered by these films and, as a result, some areas of the semiconductor surface have little or no protective covering. Contamination of the semiconductor chips at or near these uncovered areas is very likely.
It is known that silicon nitride films provide not only an electrical barrier but also a barrier to sodium as well as moisture. One of the problems of using a CVD process to deposit silicon nitride films is that the temperature range used is generally 700 to 900 degrees C. This poses a problem because aluminum metallization has a melting point of approximately 660 degrees C. Still further, CVD-deposited films have relatively high tensile stresses and, consequently, they tend to crack if made thicker than a few thousand angstroms.
Various publications indicate methods of depositing silicon nitride at temperatures below 450 degrees C., using an rf plasma to provide some of the activation energy used for the reaction of silane and ammonia or silane and nitrogen, or a combination of silane, ammonia and nitrogen. Other publications show the use of a quartz tube, and inductively coupled plasma, and gases flowing at a pressure of approximately 100 microns. These conditions generally result in poor uniformity of film layer thickness, from semiconductor wafer to semiconductor wafer, and of inadequate step coverage.
The IBM Technical Disclosure Bulletin of July, 1967, Vol. 10, No. 2, p. 100, discloses the use of an rf reactive sputtering technique for depositing a layer of silicon nitride by utilizing a silicon cathode and a gaseous mixture of ammonia and argon. The publication points out the advantage of using ammonia over pure nitrogen. However, the resulting silicon nitride film tends to have poor step coverage and the relatively high power utilized in conjunction with the low pressure required for sputtering can result in X-rays which tend to damage the semiconductor devices. The difficulties of this kind of system are clearly pointed out in U.S. Pat. No. 3,565,674, lines 6-11.
U.S. Pat. No. 3,757,733, Reinberg, describes a radial flow reactor which is utilized to deposit silicon-nitrogen films on semiconductor wafers. The apparatus described represents an improvement over prior art apparatus and illustrates that contrary to prior thinking, rf plasma deposition of silicon-nitrogen films can be achieved without undue complexity and expense. The gases described for use in Reinberg's apparatus are silane and nitrogen contained in the carrier gas argon. One of the problems we have experienced with this system is that the films produced have relatively low densities and high tensile stresses. This combination of low density and high tensile stress leads to a tendency for the films to crack during subsequent relatively high temperature processing steps as are normally required during the attaching of the semiconductor chip to a lead bearing package. These characteristics tend to limit the useful thickness of the film to a few thousand angstroms in order to prevent excessive cracking and it is often desirable to have thicker films for good coverage over the steps in the semiconductor chip.
It has been found that the use of ammonia with silane and nitrogen tends to improve the quality in the resulting silicon-nitrogen film; however, film cracking still is a serious problem.
It would be desirable to be able to produce a silicon-nitrogen film on the surface of semiconductor devices which provides protection against handling, which has good electrical isolation, step coverage and good resistance to cracking upon heating, and which can be deposited to thicknesses of approximately 1 micron without any resulting cracking.